Chemistry and crystal growth of plant wax tubules of lotus (Nelumbo nucifera) and nasturtium (Tropaeolum majus) leaves on technical substrates

Waxes consisting of hydrophobic and crystalline microstructures form multifunctional surfaces on a great number of plants. Scanning electron microscopy (SEM) and chemical analysis of extracted leaf waxes of lotus (Nelumbo nucifera Gaertn.) and nasturtium (Tropaeolum majus L.) showed that waxes of both species form small tubules and are composed of a mixture of aliphatic compounds, principally nonacosanol and nonacosanediols. Atomic force microscopy (AFM) and SEM were used to study the recrystallization of these wax tubules on technical surfaces. AFM studies provided consecutive images of the tubule formation process showing that precursor morphologies form a circular basis, which form tubules by continuous accumulation of new wax molecules. Recrystallization experiments showed wax crystal morphologies similar to those on plant surfaces, but great differences were found in tubules orientation and spatial distribution on the different substrates used. On silicon substrates, waxes preferably formed thick crusts, with randomly orientated tubules on top. On highly ordered pyrolytic graphite (HOPG) tubules predominantly grew vertically to the substrate in an ordered spatial distribution. Furthermore, on HOPG the recrystallized waxes decreased the substrate wettability with approximately 40 degrees for lotus and 30 degrees for nasturtium waxes.